Modern day internal combustion engine manufacturers, particularly diesel engine manufacturers, are concerned with meeting current and future emissions standards without negatively impacting engine power or fuel consumption. Thus, there is great interest in developing methods and systems that may be used to reduce various types of engine emissions, including, but not limited to nitrous oxides (NOx). It may be particularly desirable to improve NOx performance without negatively impacting brake specific fuel consumption (BSFC).
Control over the capture or “after-treatment” of unwanted emission gases, such as NOx, may be a function of the exhaust temperature. Modern diesel engines are provided with emission control devices designed to reduce these unwanted components of the exhaust. These emission control devices are designed to operate most efficiently in a specified range of temperatures, typically in the range of 250 to 450 degrees Celsius. If the exhaust temperature is too far above or below this range, the emission control device may be less efficient, and as a result, allow more unwanted emissions to pass through the exhaust system to the atmosphere. Accordingly, it is desirable to maintain the exhaust gas temperature within the specified range throughout various engine operating conditions.
It is common, however, for engine exhaust temperature to vary in accordance with varying engine operating conditions. For example, exhaust temperature usually varies proportionally with engine load—the higher the engine load, the higher the exhaust temperature. Accordingly, it is typically at low engine load that the exhaust temperature may fall below the range that is desired for operation of the emission control device that limits unwanted emissions. Thus, there is a need to maintain the exhaust temperature in the desired range for operation of the emission control device across as broad a range of engine load conditions as possible.
The NOx component of diesel engine exhaust may be produced as a result of excess oxygen in the combustion mixture and/or high combustion temperatures. The combustion mixture is largely made up of air, and thus contains a large amount of nitrogen. At higher combustion temperatures, the nitrogen may combine with any free oxygen that remains after combustion to produce NOx. The production of NOx may be reduced by limiting the combustion temperature, and/or limiting the amount of excess oxygen in the engine cylinder during the combustion cycle. In fact, limiting the amount of oxygen in the cylinder may also help to reduce the combustion temperature because the reduction of oxygen tends to produce less complete combustion, which in turn may reduce combustion temperatures. Thus, there is a need to limit combustion temperatures and/or the amount of excess oxygen in the combustion chamber during the combustion cycle.
Control over the exhaust gas temperature in the cylinder and the exhaust system, as well as the amount of oxygen in the combustion mixture, may be exercised through control of the intake and exhaust engine valve timing. Some internal combustion engines are to be equipped with variable valve actuation (VVA) systems designed to provide control over the timing of intake and exhaust engine valve operation. Examples of such VVA systems are provided in Vorih et al., U.S. Pat. No. 6,510,824 (Jan. 28, 2003), entitled “Variable Lost Motion Valve Actuation and Method;” and Vanderpoel et al., U.S. Pat. Appl. Pub. No. U.S. 2003/0221663 A1 (Dec. 4, 2003) entitled “Compact Lost Motion System for Variable Valve Actuation,” both of which are incorporated herein by reference. When appropriately designed and controlled, VVA systems may enable the opening and/or closing times of an engine valve to be adjusted during operation of an engine. This adjustment may be used to control various engine performance characteristics, such as cylinder temperature, exhaust system temperature, and oxygen content of the combustion mixture.
Exhaust gas recirculation (EGR) has been proposed as one method of controlling the cylinder and exhaust system temperature, as well as the oxygen content of the cylinder. Specifically, EGR has been proposed as a means of enabling diesel engine manufacturers to lower NOx emissions and meet new emission standards. Broadly speaking, using VVA to carry out internal exhaust gas recirculation requires opening an exhaust valve during an intake stroke or opening an intake valve during an exhaust stroke to facilitate the flow of exhaust gas back into the engine cylinder (or intake manifold) for future combustion cycles. The retention or flow of exhaust gas back into the cylinder for a subsequent combustion cycle may reduce the relative proportion of oxygen in the cylinder that is available for combustion. In turn, this may reduce the amount of oxygen that is available to combine with nitrogen and produce NO. Additionally, the recirculation of the exhaust gases has the effect of increasing the heat capacity of the burned gases for a given quantity of heat release, thus lowering the combustion temperature which also tends to reduce the production of NOx.
EGR may not be ideal for all engine operating conditions, however. An engine operating at a full load and at a low engine speed (such as around the peak torque operating point) is the most difficult mode for introducing EGR without losing engine power or creating a black smoke problem. Thus, there remains a need for alternative solutions.
Accordingly, it is a goal, but not a requirement, of one or more embodiments of the present invention to improve certain engine performance characteristics, including reduction of NOx, production of a tighter exhaust temperature profile across varying operating conditions (including full load), and/or improved brake specific fuel consumption across varying operating conditions (including full load). Additional advantages of various embodiments of the invention may be ascertained, in part, from the description that follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the embodiments of the invention.